1. Field of the Invention
The present invention relates to a touch panel, especially to a capacitive touch panel and the capacitance sensing apparatus and method for the same.
2. Description of Prior Art
Touch panels have extensive applications such as ATM, selling kiosk and industrial control system. The touch panel has gained more market as the popularization of smart phone and PDA, which also employ touch panel as input device for layman user.
The major touch panels include resistive type and capacitive type touch panels in terms of operation principles. More particularly, the resistive type senses a voltage corresponding to a pressing by finger or stylus. The capacitive type touch panel senses capacitance change caused by a touch of user finger, which draws little amount of current from the touch panel. The capacitive type touch panel can be further classified into surface capacitive touch panel and projected capacitive touch panel, where the projected capacitive touch panel becomes more attractive due to the realization of multi-touch function. The projected capacitive touch panel forms an In-Cell Multi-Touch Panel when it is integrated into a LCD screen and the thickness of the LCD screen is not significantly increased. In TFT LCD, indium tin oxide (ITO) is frequently used to lock storage charge and the ITO can also be used as sensor for high-density sensing when the TFT LCD is integrated with touch panel. Capacitance sensing apparatus is extremely important for TFT LCD with touch panel function because TFT induces considerable noise.
FIG. 1 shows a prior art projected capacitive touch panel 200A, which comprises a projected capacitive touch panel unit 300A and a controller 30A. The projected capacitive touch panel unit 300A comprises an insulating base (not labeled), longitudinal stripe electrodes 32A, and transverse stripe electrodes 34A, where longitudinal stripe electrodes 32A and transverse stripe electrodes 34A are perpendicular to each other. It should be noted that FIG. 1 shows only a simplified projected capacitive touch panel 200A, the longitudinal stripe electrodes 32A and transverse stripe electrodes 34A are on opposite faces of the insulating base, and the number thereof are M and N, respectively. Therefore, the controller 30A has (M+N) input lines and detects a touch position on the projected capacitive touch panel unit 300A by sensing capacitance variation between electrodes through the longitudinal stripe electrodes 32A and transverse stripe electrodes 34A.
FIG. 2 shows a related art capacitance sensing circuit 100A in a projected capacitive touch panel, where the capacitance sensing circuit 100A can be built-in the controller 30A shown in FIG. 1 to sense capacitance variation for the controller 30A, therefore the controller 30A can detect touch position. Provided that Cs denotes the contact capacitance caused by a touch on the Kth longitudinal stripe electrode 32A and Cp denotes stray capacitance, the capacitance sensing circuit 100A charges a charge Q to the contact capacitance Cs and stray capacitance Cp through a charging current source Ic. Afterward, the capacitance sensing circuit 100A first discharges the contact capacitance Cs and stray capacitance Cp through a larger discharging current source 20A and then discharges the contact capacitance Cs and stray capacitance Cp through a smaller discharging current source 22A. According to formula Q=C×V, the voltage at connection node P changes when capacitance (overall effect of the contact capacitance Cs and stray capacitance Cp) is changed by finger touch. Whether a touch is present on the Kth longitudinal stripe electrode 32A can be identified by comparing the voltage at connection node P and a reference voltage with a comparator 50A.
More particularly, the controller 30A periodically turns on the charging switch SW_C, the first discharging switch SW_A, and the second discharging switch SW_B, respectively, and the comparator 50A outputs pulse width modulation (PWM) signal indicating whether a touch is present on the Kth longitudinal stripe electrode 32A and the touch position. The controller 30A judges whether a touch is present on the Kth longitudinal stripe electrode 32A and the exact touch position by reading the PWM output signal from the comparator 50A. The capacitance sensing circuit 100A in turn conducts measurement on the longitudinal stripe electrodes 32A and transverse stripe electrodes 34A, whereby the controller 30A knows capacitance variation on the electrodes and the exact touch position on the touch panel.
However, there are two drawbacks in above-mentioned related art capacitance sensing circuit:
1. The stray capacitance Cp is larger than the contact capacitance Cs, generally being 1000 times of the contact capacitance Cs. The measuring accuracy is influenced when noise is imposed on the stray capacitance Cp. When the touch panel is employed in LCD screen, stray capacitance Cp is very large and the contact capacitance Cs is difficult to measure because the ITO electrode in the LCD screen is very close to the DCVCOM electrode. Moreover, the capacitance measurement is also hindered by noise coupled from the capacitance sensing circuit 100A on lower plate to the stray capacitance Cp on the upper plate of LCD screen.
2. The wiring is complicated because (M+N) signal lines are necessary to measure signals from the M longitudinal stripe electrodes 32A and the N transverse stripe electrodes 34A.